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In your sentence we have subjet(the contractor)+modal(shall)+be+But here we need past participle of the verb and " responsible" is not a past participle ,it 's not even a verb.

* This is an example for a modal in the passive voice :All library books must(should/may/might/..) bereturned to the library
before July 1st.

For the second sentence "The Contractor shall design and implement..).It's also a sipmle sentence to indicate a future action.Why ? becasu of the using of the modal "shall".
If you want to use passive voice, the sentence should be :
A flow diversion shall be designed and implemented by the constractore to ensure..

I hope that I was helpful...

Remember I could be easily wrong, so that you should wait for the other members to reply on you..I'm just a beginner trying to help as mutch as I canTake care ........